《消化系统》word版.doc

Chapter Three The Digestive System1. Composition of The digestive SystemAnatomically and functionally, the digestive system can be divided into tubular digestive canal (or alimentary canal) and digestive glands. The digestive canal is approximatelly 9m long and extends from the mouth to the anus. It passes the thoracic cavity and enters the abdominal and pelvic cavity. The digestive canal consists of the oral cavity, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, and ileum), and large intestine. Clinically, the part from the oral cavity to the duodenum is referred to as the upper digestive tract；that from the jejunum to the anus is called the lower digestive tract. The digestive glands secret digestive juice and are subdivided into the greater and lesser digestive glandsThe greater digestive glands include the major salivary glands，liver and pancreasThe lesser digestive glands locate within the wall of the digestive canal，such as the gastric glands and intestinal glands2. Principal Function of The Digestive SystemThe digestive system includes the organs listed in table 3-1 and illustrated in figure 3-1. The functions of the digestive system are to:A. Ingest the food;B. Break it down into small molecules that can cross plasma membranes;C. Absorb these nutrient molecules;D. Eliminate nondigestible wastes.Table 3-1: Organs of the digestive systemOrganSpecial FeaturesFunctionsMouthPharynxEsophagusStomachSmall intestineLarge intestineAnusTeeth, tongueTongueSalivary glandsGastric glandsIntestinal glandsVilliChewing of food.Formation of bolus.Digestion of starch.SwallowingPassagewayFood storage; acidity kills bacteria.Digestion of protein.Digestion of all foods.Absorption of nutrients.Absorption of water; storage of nondigestible remains.DefecationFigure 3-1. Major organs of the digestive tract.Section 1: Digestive Canal1. MouthThe mouth receives the food in its oral cavity. The taste of food results from the stimulation of chemoreceptors in taste buds on the tongue and in the nasal cavities. Therefore, smell is a very important component of taste perception.The roof of the mouth has two parts: An anterior hard palate separates the oral cavity from the nasal passages, and a posterior soft palate separates the oral cavity from the nasopharynx. The hard palate contains several skull bones-namely, portions of the maxillae and palatine bones. The soft palate is a muscular structure and ends in the uvula, a cone-shaped process. The tongue is made up of skeletal muscle covered by a mucous membrane. Intrinsic muscles that have their origin and insertion in the tongue itself change the shape of the tongue. Extrinsic muscles insert into the tongue but have their origin outside the tongue, such as on a skull bone. The extrinsic muscles move the tongue about and account for its maneuverability. A fold of mucous membrane (lingual frenulum) on the underside of the tongue attaches the tongue to the floor of the mouth. If the frenulum is too short, the individual cannot speak clearly and is said to be tongue-tied. The floor of the oral cavity and underside of the tongue are richly supplied with blood vessels, and soluble medications will enter the circulation directly if placed beneath the tongue. There are three pairs of salivary glands called the parotid, sublingual, and submandibular glands. The parotid glands are located in front of and below the ears. Each parotid gland has a duct that opens on the inner surface of the neck, just at the location of the second upper molar. Mumps, an acute viral infection of the parotid salivary glands, causes them to swell, making it difficult to open the mouth. The sublingual glands lie beneath the tongue proper, and their ducts open into the floor of the oral cavity. The submandibular glands lie in the posterior floor of the oral cavity beneath the base of the tongue. The ducts from the submandibular glands open on either side of the lingual frenulum. The salivary glands produce about 1 litler of saliva a day, which enters the mouth by way of their ducts. Saliva contains mucus and a digestive enzyme called salivary amylase. Mucus binds and lubricates the food so it is easier to swallow, and salivary amylase begins the process of digesting the food by breaking down starch. Saliva also contains chemicals that protect against bacterial infection. The teeth carry out mastication (chewing of food), the tongue assists in mastication by moving the food between the teeth. Mastication breaks down the food into small portions, making the working of the digestive enzymes more efficient. The tongue forms the chewed food into a small mass called a bolus in preparation for swallowing.The salivary glands produce saliva, which enters the mouth, where teeth masticate the food and the tongue forms a bolus prior to swallowing.2 PharynxSwallowing, a reflex action that moves food into the esophagus, occurs in the pharynx, which is a funnel-like muscular tube that opens into the nose, mouth, and larynx, ends in esophagus. It in front of vertebrae cervicales, 12cm in length. Pharynx extends from the base of the skull to the level of 6th cervical vertebra, where it becomes continues with the esophagus. Pharynx can Subdivided into nasopharynx, oropharynx, laryngopharynx, by the soft palate and epiglottis.The air and food passages merge in the pharynx. When a person swallows, the air passage is usually blocked off, and food must enter the esophagus. Figure 3-2. Sagittal section, showing the nasal cavity, mouth, pharynx, esophagus, and trachea.3 Esophagus The esophagus is a long muscular tube, connects with pharynx (about the sixth vertebrae cervicale level), ends in stomach (at the left of the eleventh vertebrae thoracale), 25cm in length.Esophagus descends from the neck into the thorax front the vertebral column and behind the trachea. At the level of the sternal angle, it passes posterior to the left principal bronchus, and then descends along the right side of the thoracic aorta. The lower part of the esophagus crosses in front of the thoracic aorta to its left side, pierces the esophageal hiatus of the diaphragm into the abdominal cavity and becomes continuous with the cardia of the stomach. So, esophagus is divided into three parts: the cervical, thoracic, and abdominal portions. (as see fig.3-3.) Along its full length, there have three constrictions of the esophagus: The first narrowing site at the commencement; the second at the point where it is crossed by the left principal bronchus; and as it pierces the diaphragm reduced the third. Tumors and foreign matters always take place in these constrictions.Figure 3-3. Position and constrictions of the esophagus.4 StomachThe stomach is a dilated sac of the upper digestic canal. Its volume is about 1000-3000mL in adult and about 30mL in newbone children, respectively. The shape, size and position of the stomach depend upon the degree of its fillings.4.1 Shape of the stomach The stomach is a thick-walled, J-shaped organ that lies in the upper left quadrant of the abdominal cavity beneath the diaphragm. It is continuous with the esophagus superiorly and the duodenum of the small intestine inferiorly. (as see fig.3-4,5)4.2 Parts of the stomach4.2.1 The stomach has two walls(anterior and posterior wall). The anterior wall faces anterosuperiorly; the posterior wall faces posteroinferiorly.4.2.2 Two openings of the stomach: the inlet is called cardia and outlet is pylorus.4.2.3 The stomach presents two curvatures: the lesser curvature is short, its concave surface looks upward and to the right. The greater curvature is longer, convex, faces downward and left.Ulcers and tumors often erode the stomach at the the lesser curvature, near the pylorus.4.2.4 Four parts of the stomach: cardiac part; fundus; body; pyloric part.4.3 Position of the stomachWhen the stomach is middle filling, the greater part of the stomach lies in the left hypochondiac region, the lesser part in the epigastric region.The cardia lies to the left of the 11th thoracic vertebra; The pylorus lies to right of the first lumbar vertebra.4.4. Gastric functions: Ingest foods; Secret gastric juice; primary digest chyme.The stomach expands and stores food. While foods is in the stomach, the stomach churns, mixing food with the acidic gastric juice.Figure 3-4. A coronal section of the stomach showing the principal regions and structures.Figure 3-5. The parts of the stomach.5 Small Intestine The small intestine is named for its small diameter, it averages about 3 meters(the developed countries)/4-5meters(the developing countries) in length, compared to the large intestine, which is about 1.5meters in length. (see fig.3-6)The small intestine is found in the central and lower portion of the abdominal cavity, where it is supported by a fan-shaped mesentery. It receives secretions from the liver and pancreas, chemically and mechanically breaks down chyme, absorbs nutrient molecules, and transports undigested material to the large intestine. The small intestine consisting of the duodenum, jejunum, and ileum. The duodenum is the first 25cm, which has four parts: the superior, descending, horizontal, and ascending parts. The jejunum forms approximately the upper 2/5,and situated above, to the left; The ileum the lower 3/5,and situated the umbilical, right inguinal region.Figure 3-6. The duodenum and pancreas6 Large Intestine The large intestine begins in the lower right quadrant of the peritoneal cavity, ends in anal canal. It has about 1.5meters. The large intestine absorbs water and electrolytes. It also prepares and stores nondigestible material (feces). The 3 histologic characters can differ the the large intestine from the small intestine. The large intestine includes the cecum, colon, rectum, and anal canal. Except the appendix, rectum, and anal canal, the large intestine has three distinctive features: the colic bands (taeniae coli), haustra coli, epiploic appendices. On the surface of the intestines have three long lines called taeniae coli, which are formed by the longitudinal muscularis; Because the taeniae coli shorter the intestinal tube, makes the intestine form many muscular sac, called haustra coli; Many fatty spheres attached to the intestine surface form appendices epiploicae. The colon is subdivided into ascending, transverse, descending, and sigmoid parts. The ascending colon is about 15cm long and extends superiorly from the cecum to the right (or hepatic) colic flexure that lies under the liver; The transverse colon is about 50cm long, it extends from the right colic flexure, directs to the left and terminates at the left (or splenic) colic flexure in contact with the spleen; The descending colon is about 20cm long, it commences at the left colic flexure, descends along the left side of the posterior abdominal wall and into the left iliac fossa where it continous with the sigmoid colon; The sigmoid colon is approximately 40 to 50 cm long, it passes down from the left iliac crest to the 3rd piece of the sacrum where it continous with the rectum.Figure 3-7. A segment of the colon, showing characters of the cecum and colon.Figure 3-8. The cecum and vermiform appendix.个Section 2: Digestive Glands1 LiverThe liver which is the largest gland in the body, lies mainly in the right upper quadrant of the peritoneal cavity, under the diaphragm. There are two main lobes, the right lobe and the smaller left lobe, which crosses the midline and lies above the stomach. These two lobes are separated by the falciform ligament, which secures the liver to the anterior abdominal wall and the diaphragm. The inferior border of